Fiber optic, current sensors, based on the Faraday effect, have a number of advantages for remotely measuring electrical currents. These include wide dynamic range, fast response, immunity to electromagnetic interference, small size, and low cost. Consequently, a variety of fiber optic, current sensors have been investigated in recent years. Mainly, they have employed a single mode, optical fiber (SMF) of clad silica.
These sensors have not yet reached the stage of practical field use due to lack of accuracy and stability. This is mainly due to intrinsic and induced, linear birefringences that degrade the contrast when the Faraday rotation method is used. A particular problem arises from the fact that these birefringences are easily generated in sensing elements employing silica fibers.
The Faraday effect is a phenomenon by which a linear, polarized light will rotate when propagating through a transparent material that is positioned in a magnetic field in parallel with the magnetic field. The size of the rotation angle (.theta.), given in degrees, is defined as EQU .theta.=VHL (1)
where H is the strength of the magnetic field (A/m), V is the Verdet constant of the material, and L is the path length over which the magnetic field acts (m).
The magnetic field strength is measured in terms of Amperes (A) times turns (T) per unit length (AT/m) where m is meters. Since values are expressed in terms of one turn, this factor is usually implicit, rather than explicit. Hence, the strength is customarily given in amperes (A), or kiloamperes (kA), per unit path length in meters (m).
The Verdet constant, V, is the angle of rotation divided by the magnetic field strength per unit length. The angle may be expressed in any of the customary units for angle measurement, but degrees are used here. Verdet constant values, unless otherwise indicated, are given in terms of degrees divided by field strength expressed as (kA.times.T/m)m.
The magnitude of the magnetic induction (B) around an infinite, straight conductor is given by the expression: EQU B=(.mu..sub.o /4.pi.)(2I/a) (2)
where I is the current, .mu..sub.o is permittivity of free space, and a is the radial distance of the magnetic field from the conductor. The magnetic field is related to the magnetic induction by the simple relation: EQU B=.mu..sub.o H. (3)
Combining equations 1 through 3 gives a proportional relation between the rotation and the current such that: EQU .theta.=VI (4)
where .theta. is in degrees, V is the Verdet constant, and I is in kiloamperes (kA). Thus, the sensitivity of a method for measuring a current depends on how accurately the angular rotation can be measured.
The degree of sensitivity in measuring the angular rotation is influenced by another factor; birefringence. Birefringence arises primarily from stresses that result from bending, or otherwise distorting, a fiber in its disposition. The sources of linear birefringence in single mode fibers include residual stress from fabrication, bending, contact, and thermal stresses (Yamashita et al., "Extremely Small Stress-optic Coefficient Glass Single Mode Fibers For Current Sensor", Optical Fiber Sensors, Sapporo Japan, paper We2-4, page 168 (1996) ("Yamashita").
The stress-induced birefringence is quantified in terms of a coefficient, called the photoelastic constant (or the photoelastic coefficient). The photoelastic coefficient (B.sub.p) may be defined as the coefficient relating the difference in the refractive indices in the stress direction (n(par)) and in the perpendicular direction (n(per)), to the magnitude of the applied stress: EQU n(par)-n(per)=B.sub.p .sigma. (5)
It may also be regarded as the phase shift measured in units of wavelength in nanometers (nm) per path length in centimeters (cm) divided by the stress in kilograms per square centimeter (kg/cm.sup.2). The values then are in units of (nm/cm divided by kg/cm.sup.2).
An ideal glass fiber will have a photoelastic coefficient of zero, thereby nullifying any effect of stress-induced birefringence. However, this has proven difficult to obtain in conjunction with an appropriate Verdet constant. Certain applications require that a current sensor must be designed to provide a single, accurate value for a current regardless of current intensity, or total length of the sensor.
Current measurement during normal, stable operations presents no special problem. However, a problem can arise when it becomes necessary to measure an exceptionally large current. Such a current may arise, for example, from a surge or fault caused by a line failure, such as a short circuit.
In measuring such a current, unless a fiber has a small Verdet constant, there is the risk of creating an angle of rotation greater than 90 degrees. In that case, the angle will register the same as an angle of less than 90 degrees, thus bringing the measurement into question. This is particularly serious where a double pass measurement is made.
It is necessary, then, that the angle of rotation imparted to polarized light by a current passing through a fiber sensor does not exceed 90 degrees. Actually, it is desirable that the angle not even approach 90 degrees regardless of current intensity. An acceptable glass fiber sensor must meet this requirement in the event the current being measured is exceptionally large.
It is a purpose of the present invention to provide a novel, glass fiber sensor for measuring electrical currents.
Another purpose is to provide a glass having a combination of properties that make the glass useful to function as such novel sensor.
A further purpose is to provide a modified, heavy flint glass having the combination of properties required for use in the sensor for a current measuring device.
A still further purpose is to provide a method of reducing the Verdet constant value of a heavy flint glass to permit accurate measurement of an electrical current even when that current is exceptionally large.
Another purpose is to provide a novel composition for the core of a glass fiber to be used in the sensor of a measuring device for electrical currents.